How to resize and compress an image on a Mac

Resize vs compress

Two different things make a picture “smaller”:

• Resizing changes the number of pixels (for example 4032 × 3024 down to 1200 × 900). The file gets much smaller and the picture stays sharp at its new size.
• Compressing keeps the pixels but saves the JPG at a lower quality, so the file takes fewer KB.

For the smallest file that still looks good, resize first and compress a little. Preview does both.

Resize an image in Preview

1. Open the picture in Preview (double-click it, or right-click → Open With → Preview).
2. Click Tools → Adjust Size.
3. Make sure Scale proportionally and Resample image are ticked.
4. Pick the unit next to Width and Height: pixels, percent, cm, mm or inches.
5. Type the new width; the height follows. The bottom of the box shows the resulting size.
6. Click OK, then File → Save.

Saving replaces the original. To keep it, use File → Export instead and give the copy a new name.

Resize to cm in Preview

Preview can work in cm directly. Set Resolution to the DPI your form asks for (for example 200 pixels/inch), choose cm, and type the size, such as 3.5 × 4.5. At 200 DPI that gives 276 × 354 pixels. Most photos have a different shape from the form, so crop first: drag a selection over the part to keep and press Command + K (Tools → Crop).

Compress an image in Preview (lower the KB)

1. With the picture open, click File → Export.
2. Set Format to JPEG.
3. Move the Quality slider to the left. Preview shows the File Size as you move it.
4. Click Save.

The same Export box also turns a HEIC photo into a JPG. Moving the slider all the way down makes photos look blocky; it is usually better to resize to fewer pixels and keep the quality slider around the middle.

Resize many images at once

In Preview: select all the pictures in Finder and open them together. In Preview’s sidebar press Command + A to select them all, then Tools → Adjust Size. The new size applies to every picture.

In Finder (macOS Monterey and later): select the pictures, right-click → Quick Actions → Convert Image. Choose the format (JPEG, PNG or HEIF) and an Image Size (Small, Medium, Large or Actual Size), then Convert to JPEG. Finder saves converted copies next to the originals. Untick Preserve Metadata if you do not want to keep details such as the location.

Export a smaller copy from Photos

1. In the Photos app, select the photos.
2. Click File → Export → Export Photos.
3. Set Photo Kind to JPEG, choose a JPEG Quality, and a Size (Small, Medium, Large, Full Size or Custom with a maximum width or height).
4. Click Export and pick a folder.

When you need an exact KB size

Preview shows the file size, but reaching a limit like 50 KB or a range like 20 to 50 KB still means trying sizes and qualities until one fits. Our image compressor does that for you: type the limit and it picks the largest size and cleanest quality that stay under it. For forms that give the size in cm with a DPI and a KB range, use resize image in cm. Both run in your browser, so your pictures never leave the Mac.

Common questions

Does resizing in Preview lower the quality?

Making a picture smaller with Resample image on keeps it sharp at its new size. Each time you save a JPG, a little detail is lost, so resize from the original once rather than many times in a row.

Why does Preview show a different size than Finder?

The size in the Adjust Size box describes the picture in memory, not the saved file. The size shown in the Export box, and in Finder after saving, is the one a form checks.

How do I stop my Mac from making HEIC files?

Your Mac does not create them; your iPhone does. On the iPhone go to Settings → Camera → Formats → Most Compatible. To convert HEIC photos you already have, use Preview’s Export or our HEIC to JPG converter.